Manganese is an electrochemically very base metal. Since a continuous manganese-electroplating or manganese-alloy-electroplating of a steel sheet results in the production of a hydrogen gas in a plating solution, a plating efficiency of the manganese-electroplating or the manganese-alloy-electroplating is limited to about 40 to 85%.
When industrially applying the manganese-electroplating or the manganese-alloy-electroplating, manganese ions or manganese alloy ions in the manganese electroplating solution or the manganese alloy electroplating solution are electrochemically reduced into metallic manganese or a metallic manganese alloy, which is taken out the manganese electroplating solution or the manganese alloy electroplating solution, thus causing the decrease in the concentration of manganese ions or manganese alloy ions in the manganese electroplating solution or the manganese alloy electroplating solution. It is thus necessary to keep the concentration of these ions within a certain range. In order to keep a constant concentration of manganese ions or manganese alloy ions in the manganese electroplating solution or the manganese alloy electroplating solution, it is necessary to constantly replenish the manganese electroplating solution or the manganese alloy electroplating solution with manganese ions or manganese alloy ions.
For the purpose of replenishing the electroplating solution with ions of a metal or an alloy for plating when continuously electroplating a steel sheet, a general conventional practice comprises using a metal or an alloy for plating as a soluble anode, and causing a DC electric current to flow between the soluble anode and the steel sheet to be plated, thereby forming a metal plating layer or an alloy plating layer on the surface of the steel sheet.
However, since, when using a metal having a plating efficiency of almost 100% such as copper or zinc as the soluble anode, the amount of metal ions taken out the plating solution for the formation of the plating layer on the surface of the steel sheet is substantially in equilibrium with the amount of metal ions supplied from the soluble anode into the electroplating solution, the concentration of metal ions in the electroplating solution is kept substantially at a constant level.
When using manganese or a manganese alloy as the soluble anode, in contrast, the plating efficiency of manganese or a manganese alloy is so low as 40 to 85% as compared with copper and zinc. Therefore, the amount of manganese ions or manganese alloy ions supplied from the soluble anode into the manganese electroplating solution or the manganese alloy electroplating solution is larger than the amount of manganese ions or manganese alloy ions taken out the manganese electroplating solution or the manganese alloy electroplating solution through the manganese-electroplating or the manganese-alloy-electroplating of the steel sheet. As a result, there occurs the increase in the concentration of manganese ions or manganese alloy ions in the manganese electroplating solution or the manganese alloy electroplating solution.
For this reason, in order to keep a constant concentration of manganese ions or manganese alloy ions in the plating solution, it is necessary to reject part of the plating solution from the plating tank, add water to the plating tank to dilute the plating solution, and thus to reduce the concentration of manganese ions or manganese alloy ions. This requires not only a waste of the expensive plating solution, but also a cost for rejecting the plating solution, thus making it economically unfeasible to practice this plating.
Therefore, when continuously manganese-electroplating or manganese-alloy-electroplating a steel sheet, an insoluble anode must be used.
However, manganese ions are usually present in the divalent form (Mn.sup.2+) in many cases in a manganese electroplating solution or a manganese alloy electroplating solution. When the plating solution contains a complexing agent, however, manganese ions are sometimes present in the form of trivalent or higher complex ions in the plating solution. When the manganese-electroplating or the manganese-alloy-electroplating is carried out with the use of an insoluble anode, the divalent manganese ions (Mn.sup.2+) are oxidized on the surface of the insoluble anode into trivalent or higher manganese ions in the solid state or the ionic state.
Manganese ions in the solid state and having at least trivalence are more oxidized into such a solid oxide as MnO.sub.2 or Mn.sub.2 O.sub.3, which are precipitated in the manganese electroplating solution or the manganese alloy electroplating solution, largely impairing the operating efficiency, forming an obstacle for the plating operation, and causes flaws on the manganese plating layer or the manganese alloy plating layer formed on the surface of the steel sheet, thus degrading the merchantability of the plated product. Manganese ions not taking the form of the solid oxides and having at least trivalence in the ionic state dissolve, on the other hand, the manganese plating layer or the manganese alloy plating layer formed on the surface of the steel sheet, accelerate the production of a hydrogen gas at the cathode, thus deteriorating the electrolytic efficiency, reducing the plating efficiency, and seriously degrading the productivity.
It is therefore necessary to remove these oxidized manganese (manganese in the solid state or in the ionic state and having at least trivalence is hereinafter referred to as "multivalent manganese") from the plating solution.
As a means to solve the above-mentioned problems, there is known a method for removing multivalent manganese produced in a manganese-zinc alloy electroplating solution by contact-reducing said multivalent manganese with the use of metallic zinc or metallic manganese into divalent manganese ions (Mn.sup.2+).
For example, Japanese Patent Provisional Publication No. 62-44,598 discloses a method for removing multivalent manganese produced in an electroplating solution, which comprises:
removing, when electroplating a steel sheet in a manganese-zinc alloy electroplating solution comprising manganese sulfate and zinc sulfate as main components and citric salt as a complexing agent, multivalent manganese having at least trivalence produced in said electroplating solution through the contact-reduction of said multivalent manganese with the use of at least one of metallic zinc and metallic manganese into divalent manganese ions (Mn.sup.2+), thereby recovering said plating solution (hereinafter referred to as the "prior art 1").
The above-mentioned prior art 1 is a technique useful for removal by reduction of multivalent manganese in the ionic state produced in a manganese-zinc alloy electroplating solution, and in addition, industrially favorable in that it is not necessary to install special facilities for the reduction of multivalent manganese.
The prior art 1 has however the following problems.
The production of multivalent manganese in the manganese-zinc alloy electroplating solution cannot be prevented by the prior art 1. Furthermore, the prior art 1 is practically inconvenient in that the reaction for reduction-removing multivalent manganese in the solid state in the manganese-zinc alloy electroplating solution proceeds only at a low reaction rate because it is an intersolidus reaction and it takes much time to remove multivalent manganese.
As a means to solve the above-mentioned problems, there is known a method for reducing multivalent manganese produced in a manganese electroplating solution or a manganese alloy electroplating solution, by means of a hydrogen gas with palladium (Pd) as a catalyst, into divalent manganese ions (Mn.sup.2+).
For example, Japanese Patent Provisional Publication No. 59-76,899 discloses a method for reducing multivalent manganese produced in an electroplating solution, which comprises:
reducing, when manganese-electroplating a metallic material in a manganese electroplating solution containing divalent manganese ions (Mn.sup.2+), multivalent manganese having at least trivalence produced through oxidation of manganese ions (Mn.sup.2+) in said electroplating solution by means of a hydrogen gas activated by palladium or a palladium alloy (hereinafter referred to as the "prior art 2").
The above-mentioned prior art 2 has the following problems.
The production of multivalent manganese in the manganese electroplating solution cannot be prevented by the prior art 2. It is necessary, in the prior art 2, to use expensive palladium as the catalyst, consume a hydrogen gas which is not usually used for the manganese-electroplating or the manganese-alloy-electroplating, and install facilities for the reduction of multivalent manganese. The prior art 2 is uneconomical and industrially disadvantageous in that the cost for installing such facilities is required.
Both the above-mentioned prior arts 1 and 2 are to reduce multivalent manganese produced in the manganese electroplating solution or the manganese alloy electroplating solution into divalent manganese ions.
Under such circumstances, when using a manganese electroplating solution or a manganese alloy electroplating solution, using an insoluble anode, and causing a DC electric current to flow between the insoluble anode and a steel sheet during travelling through the electroplating solution while replenishing the manganese electroplating solution or the manganese alloy electroplating solution with manganese ions or manganese alloy ions, thereby forming a manganese plating layer or a manganese alloy plating layer on at least one surface of the steel sheet, there is a strong demand for the development of a method which does not cause the production of multivalent manganese in the manganese electroplating solution or the manganese alloy electroplating solution, but such a method has not as yet been proposed.